Soul Arrow now stacks with +Attack Arrows.

Well...If you can prove it's not actually working - go ahead.

Here's a reasonably controlled experiment:

Go pap. Any high leveled enough BM can solo pap. Just time it.

Control 1: With Soul Arrow and no attack arrows
Control 2: With Attack Arrows and no Soul Arrow

Test: With Attack Arrows and Soul Arrow.

Depending on which control the test matches, that is your answer.